JPH08170288A - Removal of sodium salt and potassium salt from caught ash of soda-recovering boiler - Google Patents

Abstract

PURPOSE: To provide the method for removing from a caught ash the sodium and potassium salts which are concentrated and accumulated in a pulp-digesting chemical as impurities. CONSTITUTION: The method for removing sodium and potassium salts from the caught ash of the soda-recovering boiler comprises slurrying the ash caught from the combustion exhaust gas of the soda-recovering boiler in water, controlling the pH of the slurry to <=10 by the addition of sulfuric acid, controlling the temperature of the slurry to >=20 deg.C, holding the mixture for a definite time to dissolve the sodium and potassium salts contained in the caught ash in water, cooling the slurry at <20 deg.C, separating and recovering the deposited solid contents from the liquid portion, and throwing away the left liquid portion outside the system.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is applied to a soda recovery boiler in a pulp manufacturing plant, and collects salt and potassium salts as impurities that are concentrated and accumulated in pulp cooking chemicals from combustion exhaust gas. It relates to a method of removing ash.

[0002]

2. Description of the Related Art Removal of potassium salts in pulp cooking chemicals
There is no method that has been applied to actual equipment as a conventional method, but it is similar
There is a salt removal method as the technique. That is, soda times
Uses a small amount of water and circulating liquid to collect the collected ash from the exhaust gas of the collecting boiler.
To make it into a slurry and dissolve the salt in water at a temperature of 60-70 ℃.
It is a method to separate and recover the solid content in the slurry.
However, this method can easily remove the salt in the drug.
However, since the solubility of potassium salt is low, potassium salt is removed.
Na is an effective drug when trying to leave₂SO _FourAt the same time
However, it has a drawback that the chemical loss becomes large.

[0003]

In pulp cooking chemicals, salt and potassium salts as impurities are mainly brought from pulp logs. These impurities leave the chemical recovery system along with the pulp that is the product and the lost chemical that goes out of the system, but as the chemical recovery becomes closed and the chemical recovery rate increases, it is concentrated and accumulated in the chemical recovery system. There is a tendency to

It is well known that concentrated and accumulated salt or potassium salt, as an inert substance, not only increases the circulation load of chemicals and leads to a decrease in economic efficiency due to an increase in dead load, but also significantly increases the corrosiveness of the chemical recovery system. ing. Especially in soda recovery boilers that burn pulp waste liquid,
The presence of sodium chloride and potassium salt not only enhances the corrosiveness of the heat transfer tube in the high temperature part of the boiler, but also significantly increases the adhesion of the combustion ash adhering to the surface of the heat transfer tube, increasing the flue pressure loss of the soda recovery boiler and continuing the boiler. Leading to disruption of operations.

[0005]

The present invention is a method for removing salt and potassium salts concentrated / accumulated in a pulp cooking chemicals recovery system, in which collected ash collected from combustion exhaust gas of a soda recovery boiler is The mixture is mixed with water to form a slurry, the pH of the slurry is adjusted to 10 or lower, the temperature is adjusted to 20 ° C. or higher, and the salt and potassium salt in the collected ash are dissolved in water by maintaining the slurry for 20 hours. After precipitating solids by cooling to a temperature below ℃, the solids are separated into liquids and the solids are recovered, and the liquids are discarded outside the system. It is a method of removing common salt and potassium salt.

As described above, in the conventional salt removal method, the ash collected from the soda recovery boiler is brought into contact with a small amount of water at 60 to 70 ° C. to remove the salt, but the potassium salt has a low solubility in water. Therefore, if water is added until the required amount of removal is obtained, the amount of sodium sulfate, which is an effective chemical, is increased to elute, resulting in poor economic efficiency.

Therefore, in the present invention, the main purpose is to remove potassium salts, and the collected ash is slurried with water and neutralized with sulfuric acid to adjust the pH of the slurry to 10 or less (10 to 7). Most of the carbonates contained in the collected ash mainly composed of salt are also sulfates. In this way, potassium and sodium contained in the collected ash are eluted as potassium sulfate and sodium sulfate to generate sensible heat, heat of dissolution, heat of neutralization, etc. of the collected ash, and then the slurry is washed with industrial water or the like. The temperature of 20
° C. or higher (preferably Na ₂ SO 30 ° C. before and after reduction in solubility is less in ₄₎ is adjusted to. The slurry is then treated at 20 ° C., where the solubility of sodium sulfate is low and therefore chemical loss is lower.
It is intended to precipitate sodium sulphate at a low temperature of less than 20 ° C. (generally less than 20 ° C. and 10 ° C. or higher) for separation / recovery to dissolve / remove potassium sulfate. By cooling the aqueous solution of 20 ° C. or more (preferably around 30 ° C.) in which the collected ash is dissolved to a temperature of less than 20 ° C., the degree of supersaturation of the solution can be increased and precipitation of coarse crystals can be promoted. Since the precipitated crystal grains have a large particle size, subsequent separation / recovery becomes easy.

[0008]

[Operation] Figure 1 shows Na₂SO_Four-K₂SO_FourDissolution of mixed system
It is a plot of degrees against temperature. In FIG. 1, N
a₂SO_Four(Mixed system), K₂SO_FourWhat is (mixed system) Na?
₂SO_Four-K₂SO_FourNa in mixed system₂SO_Four, K₂S
O_FourSolubility curve, Na ₂SO_Four(Alone), K₂SO_Four
(Single) means each individual Na₂SO_Four, K₂SO_Four
Shows the solubility curve of K in FIG.₂SO_FourSolubility of
Changes little with temperature, but Na₂SO_FourSolubility of
Has a large change with temperature, and its solubility is extremely small at low temperatures.
It gets worse.

Using a fixed amount of the collected ash of the same composition, 80
Make slurries at 10 ° C and 10 ° C. In this case, hydrogenation of both
Na that melts on the water side by changing the amount added₂SO_FourThe same amount of
Slurry Then, in the case of 80 ℃ slurry, it melts
Na₂SO_FourAbout 1/4 amount of K₂SO_FourOnly melted
No, but at 10 ° C, Na₂SO_FourThe same amount of K ₂
SO_FourDissolves. Therefore, K that remains in the solid matter in the slurry
₂SO_FourIs Na₂SO _FourAgainst K₂SO_FourMelts a lot
Solids in the slurry at 10 ° C are reduced as less is discharged
K inside₂SO_FourIs about 1 / compared to that at 80 ℃
It becomes 4.

However, when the collected ash of the recovery boiler is brought into contact with low-temperature water from the beginning, undissolved coarse particles are likely to remain, and slurry handling such as stirring and pumping is difficult. On the other hand, as in the present invention, when the slurry is previously adjusted to 20 ° C. or higher (preferably around 30 ° C. in which the decrease in the solubility of Na ₂ SO ₄ is small) and subsequently kept at a low temperature of less than 20 ° C. Since the ash is once in solution, it is easy to handle.

In the above operation, the dust collecting ash (electrostatic dust collecting ash) is once dissolved in water and then crystallized. During this dissolution, the dust collecting ash (hereinafter referred to as EP ash) is dissolved. salt,
Particularly, Na ₂ CO _{3 and the} like generate a large heat of solution as shown in the following equation.

Embedded image

[0012] Figure 2 but shows the pH and concentration of Na ₂ CO ₃ and Na ₂ SO ₄ in the separation liquid of the slurry separation liquid, the slurry when Na ₂ CO ₃ in EP ash increases Since Na ₂ CO ₃ dissolves in the solution as the pH rises, the Na recovery rate decreases overall. In order to prevent this, as shown in FIG. 2, it is necessary to neutralize the pH of the slurry to 10 or less, but during this neutralization, a large heat of neutralization is generated as in the following equation.

Embedded image Na ₂ CO ₃ + H ₂ SO ₄ → Na ₂ SO ₄ + H
₂ O + CO ₂ -61.6 Kcal / kg And EP ash is one collected in a temperature range around an exhaust gas temperature of about 200 ° C., and its own sensible heat is also large.

Therefore, when crystals are precipitated by suddenly making the slurry into less than 20 ° C., all of the sensible heat, the heat of dissolution, the heat of crystallization, the heat of crystallization, etc. of the EP ash are all consumed by the expensive cold heat cooled to less than 20 ° C. Need to be cooled, which increases the cooling cost.

On the other hand, as in the method of the present invention, EP ash is first dissolved and slurried in the first stage, and then neutralized at 20 ° C.
Above (preferably around 30 ℃), 20 ℃ in the second stage
The mixture is cooled to a temperature below the temperature to precipitate crystals, and solid-liquid separation is performed. In this case, the sensible heat, the heat of dissolution, the heat of neutralization, etc. of the EP ash in the first stage can be cooled by heat exchange with ordinary cooling water, and the crystallization heat in the second stage can be mainly cooled by using a cooler. It will be cooled below ℃. Thus, the cooling cost can be greatly reduced by cooling the sensible heat, the heat of dissolution, the heat of neutralization, etc. of the EP ash by cooling with cooling water such as ordinary industrial water.

The method of the present invention is then applied to Na ₂ C in EP ash.
The higher the O ₃ concentration, the greater the effect. In addition,
Since the aqueous solution of the collected ash which has been heated to 20 ° C. or higher in advance is cooled to below 20 ° C. in the next step, the supersaturation degree of the solution is increased and N
Precipitation of coarse crystals of a ₂ SO ₄ can be promoted.

[0016]

EXAMPLE A concrete example of the present invention will be described with reference to FIG.
Explain. Collected ash from the electrostatic precipitator 1.0 in the first stirring tank 1.
Tons / hour (The composition of the collected ash is wt% NaCl: 9.7
%, Na₂SO_Four: 67.2%, Na₂CO₃: 10.
1%, KCl: 1.5%, K ₂SO_Four: 9.9%, K₂
CO₃: 1.6%) continuously supplied from conveyor 2.
In parallel with that, water: 1.0 ton / in the first stirring tank 1
Supply time. Strongly stir the stirrer 3 in the first stirring tank 1.
Then, the collected ash and water are thoroughly mixed to form a slurry. On this occasion,
With the heat exchanger 4 using cooling water, the slurry in the first stirring tank 1 is
Keep the temperature at 25 ° C. At this time, the slurry
pH is Na₂CO₃Since it shows 10 or more, dilute sulfuric acid
Dilute the sulfuric acid 12 in the tank 11 with the instruction of the pH meter 14
The H regulator 15 controls the chemical injection pump 13 to control the first stirring tank 1
And the pH of the slurry is adjusted to 9.
As a result, Na in the slurry₂CO₃Most of the Na₂S
O _FourConvert to. In this state, stay for 1-2 hours,
Approximately 2.0 tons / hour of slurry is supplied to the second stirring tank 6 by the rear pump 5.
Transfer.

Next, water or ice is supplied to the second stirring tank 6 in a total amount of about 1 ton / hour and gently stirred by the stirrer 7.
8 hours allowed to stay, encourage crystal growth of Na ₂ SO _4. At this time, the temperature of the slurry in the second stirring tank 6 is kept at 15 ° C. by the supply of ice and the cooler 8.

Next, about 3.0 tons / hour of the slurry in the second stirring tank 6 was transferred to the decanter 10 by the pump 9, and the solid content in the slurry was separated. As a result, 2.1 ton / hour of sludge and 0.9 ton / hour of the separated liquid were obtained. The separated liquid 0.9 ton / hour was diluted with water and then sent to the wastewater treatment device in the factory. The 2.1 ton / hour sludge is composed of 0.9 ton / hour of solid content, 1.2 ton / hour of water of crystallization and adhering liquid. The sludge is dried if necessary and returned to the Na ₂ SO ₄ mixing tank.

The recovery of Na ₂ SO ₄ in this example is about 8
5%, the removal rate of potassium components such as KCl and K ₂ SO ₄ was about 95%, and the removal rate of NaCl was about 97%.

Next, 1 ton of EP ash containing 15% of Na ₂ CO ₃ at 150 ° C. (the rest is assumed to be Na ₂ SO ₄ : 85%) was slurried with 2 tons of water, and this was all cooled by a cooler. Table 1 shows a comparison between the amount of cooling heat when cooled to 15.degree. C. and the amount of cooling heat of the preferred method of the present invention using cooling water and a cooler.

[0021]

[Table 1]

In Table 1, the values (measured values) shown in Table 2 below were used.

[0023]

[Table 2]

As is apparent from Table 1, the method using the cooling water and the cooler according to the present invention has about half the total cooling heat amount (5
It is understood that the cooling heat amount of 9155/127855 × 100 = 46.3%) is cooled by inexpensive cooling water, and the rest is cooled by the cooler, so that the cooling cost can be greatly reduced.

[0025]

【The invention's effect】

1. By removing salt and potassium salt from the collected ash of the recovered boiler exhaust gas, it is possible to reduce the concentration of chlorine and potassium in the chemical system. As a result, the adhesion of the combustion ash of the recovery boiler flue was reduced, and the conventional continuous operation of about 3 months could be extended to 6 months to 1 year. 2. At the same time, the amount of steam consumed in the suit blow for removing the ash adhering to the heat transfer surface was significantly reduced. 3. By removing the salt and potassium salt, the melting point of the adhered ash was not lowered, and the corrosion of the heat transfer tube in the high temperature part of the boiler was reduced. The corrosiveness of each part of the recovery boiler has been improved, for example, the service life of the smelt spout has been extended about three times. 4. Cooling with cooling water cuts the power of the cooler in half, making it possible to greatly reduce cooling costs.

[Brief description of drawings]

FIG. 1 Na ₂ SO ₄ mixed system of Na ₂ SO ₄ —K ₂ SO _4
4 , solubility of K ₂ SO ₄ and Na ₂ SO ₄ alone system, K ₂
Sole system diagram showing the solubility of each of the SO _4.

FIG. 2 pH of slurry separation liquid and Na ₂ S in the separation liquid
O _4, table showing the relationship between the concentration of Na ₂ CO _3.

FIG. 3 is a schematic view of a process for explaining one embodiment of the present invention.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takayuki Maeda 1-1, Atsunoura-machi, Nagasaki-shi, Nagasaki Mitsubishi Heavy Industries, Ltd. Nagasaki Shipyard (72) Inventor Yoshihisa Arakawa 2-5-1 Marunouchi, Chiyoda-ku, Tokyo Sanryo Heavy Industries Co., Ltd.

Claims (1)

[Claims] 1. The collected ash collected from the combustion exhaust gas of a soda recovery boiler is mixed with water to form a slurry, and p of the slurry is mixed.
H was adjusted to 10 or less by adding sulfuric acid and the temperature was adjusted to 20 ° C. or more, and the salt and potassium salt in the collected ash were dissolved in water by maintaining for a certain time, and the slurry was cooled to a temperature of less than 20 ° C. A method for removing salt and potassium salt from the collected ash of a soda recovery boiler, which comprises depositing a solid content, separating the solid content and a liquid, recovering the solid content, and discarding the liquid outside the system. .